Signal processing methods are used in a wide range of applications including, for example, measuring an output from a photo-detector of an array in an optical navigation system. Optical navigation systems, such as an optical computer mouse, trackball or touch pad, are well known for inputting data into and interfacing with personal computers and workstations. Such devices allow rapid relocation of a cursor on a monitor, and are useful in many text, database and graphical programs. A user controls the cursor, for example, by moving the mouse over a surface to move the cursor in a direction and over distance proportional to the movement of the mouse. Alternatively, movement of the hand over a stationary device may be used for the same purpose.
The dominant technology used today for optical mice relies on a light source illuminating a surface, a two-dimensional (2D) array of photosensitive elements to capture the resultant images, and a signal processor that correlates successive images to detect and quantify the motion of the mouse. The image can be produced in a number of ways including illuminating the surface at or near grazing incidence to produce and image shadows due to roughness of the surface, illumination with a coherent light source to produce a speckle image of the surface, or the use of a pattern printed onto the surface itself. Regardless of the imaging method used to produce a trackable image, a processor captures the image and does a series of correlations between successive images to determine the most likely motion between frames. A similar method can be used with a linear sensor to track one dimension (1D) motion. In either case, the correlation used to track the motion of the image requires a great deal of processing and results in an unsatisfactory power consumption that limits the usefulness of the technique in power sensitive applications, such as wireless mice.
An alternative method to correlation uses an array of photosensitive elements or detectors, such as photodiodes, in which the output of the individual elements in the array are combined or wired together in a repeating pattern spanning two or more detectors to track motion along one axis or in one dimension. Generally, the detectors are wired in groups to detect of motion through movement of a light-dark pattern known as speckle. Speckle is the complex interference pattern generated by scattering of coherent light off of an optically rough surface and detected by a photosensitive element, such as a photodiode, with a finite angular field-of-view or numerical aperture. The image mapped to or captured on the comb-array may be magnified or de-magnified to achieve matching and so that the distribution of spatial frequencies in the image is roughly centered around the spatial frequencies of the array. Through use of signal processing, it is possible to track the movement of this image as it moves back and forth across the comb-array and from that tracking derive the motion of the surface relative to the array.
Although a significant improvement over prior art, these speckle-based devices have not been wholly satisfactory for a number of reasons. In particular, optical navigation systems using the above comb-array have not demonstrated the accuracy demanded in state-of-the-art pointing devices today, which generally must have a path error of less than about 1%. Furthermore, the above approach involves processing signals from multiple signal processing paths, and suffers from relatively complex signal processing requirements increasing power consumption of the device or system.
Accordingly, there is a need for a signal processor or signal processing circuit and method that is capable of tracking motion from an optical sensor in one or more directions or dimensions with an acceptable path error in any direction (i.e. on-axis as well as off-axis directions). It is desirable that the circuit and method achieve this end without increasing the complexity and power consumption of the signal processor or the optical navigation system in which it is used. It is still further desirable that the method reduces the power consumption of the system, thereby making it more suitable for power sensitive applications such as wireless mice.
The present invention provides a solution to this and other problems, and offers further advantages over conventional signal processing methods.